The invention relates to a method for conveying a pulverulent material by means of an injector, in which, after the material has been introduced into an entraining gas, dosing gas is added. The invention also relates to an application of the method for coating the weld seams of can bodies, an apparatus for conveying a pulverulent material by means of an injector, and an application of the apparatus.
It is known to convey pulverulent materials by means of an injector. In the conveying of coating powder in particular, the use of an injector in which dosing air is added after the coating powder is introduced into the entraining air is also known. FIG. 1 shows such an injector 1 in which air is injected through a jet 4 into the injector chamber 5, which has a powder inlet 6 for introducing powder into the entraining stream from a feed container which is not shown. The powder entrained in the airstream passes into the chamber 10, where dosing air is added to the powder stream through a connection 8 and a duct 9. The powder/air mixture is fed to the coating point by a hose and pipework connected to the hose coupling 11.
Control of the entraining air and dosing air, which are taken from a conventional compressed-air source, has, in the past, been effected either by separate control valves or by two valves arranged on a common shaft so that only one control knob has to be operated in order to set both valves. Especially in applications where the powder/air mixture has to travel a relatively long distance through pipework to the coating point (e.g. a distance of 1 meter or more) or where a very homogeneous powder/air mixture is required, the control of entraining air and dosing air has proved very difficult. The balance between entraining air (which determines the quantity of powder drawn from the feed container) and dosing air (which affects the velocity of the powder/air mixture in the piping and the homogeneity of the mixture) is very difficult to set correctly if the conveying distances are long and/or a high standard of homogeneity is required, and even with two coupled control valves it has been found that satisfactory control is possible only in a narrow operating range.
Particularly awkward conditions occur in the known coating of welded seams of can bodies at the end of a body welding machine. On the one hand, the injector has to send the powder along an extended line, as it has to be routed through the welding machine and along the body forming and welding path. On the other hand, to obtain a satisfactory coating of the welded seam, a constant quantity of powder, homogeneously distributed, must reach each can body as it is conveyed past the coating nozzle. The can bodies are conveyed at a rate of e.g. 18 cans per second (ie. for a standard size of can, at approximately 100 m/min), possibly leading to a large number of cans receiving an insufficient coating if any fluctuations occur, even momentarily, in the homogeneity of the powder/air mixture or in the absolute quantity of powder deposited.